Ion mobility spectrometry (IMS) is an analytical technique used to identify ionized molecules in the gas phase based on their mobility in a gas. In time-of-flight IMS, pulses of ions are accelerated by a voltage gradient along a drift tube and the migration time (drift time), which is characteristic of different ions, is recorded by a detector at the end of the drift tube. Typically, drift tubes are composed of alternating electrode (metal) and insulator (ceramic, plastic) rings.
Stacked alternating electrode and insulator designs have a number of drawbacks. Stacked drift tubes must be manufactured from machined components which are difficult and time consuming to produce, especially insulators made from machineable ceramics. Machineable ceramics are expensive, typically require dedicated equipment such as diamond tools, are more prone to breaking during machining and do not hold tolerances well when compared with traditionally machined materials (brass, aluminum, steel). The ceramic components of drift tubes are also more prone to breakage during use (e.g. if it is dropped). The wall thickness of both the electrode and insulator components needs significant wall thickness for mechanical stability. This results in a heavy and large outer diameter drift tube relative to the size of ion path (especially important in portable, handheld applications), and a high thermal mass which requires a significant time to heat to operating temperature and cool when the system is put in a non-operational state. The stacked drift tube designs also require some sort of support structure to keep all of the components in contact with each other as well as conductive enclosure surrounding the drift tube (a Faraday cage) that prohibits perturbation of the drift tube's electric field from an external potential, each of which add to the size and weight of the assembly. Stacked drift tubes also have interfaces between the various stacked components that create crevices, cracks or junctions that result in trapped analyte carrying over from one analytical run to the next. Back-diffusion of room air into a stacked drift tube can also cause contamination in the IMS drift tube.
What is needed is an ion mobility drift tube that provides improved noise immunity, ruggedness and manufacturability while reducing size, weight, and power requirements.